1. Field of the Invention
The present invention relates to a method and apparatus for detecting defects or faults in a continuously moving strip of transparent material, especially a thin glass strip or band, by means of a narrow light beam conducted through the strip transverse to its motion direction, whose scattered light originating from the defect or defects is detected, evaluated and processed to produce a control signal.
2. Description of the Related Art
Testing for faults or defects is of considerable importance in the manufacture of thin glass. Defects with a size of more than 100 Πm are not accepted by the consumers and must thus be reliably detected. To an increasing extent machine testing methods, which are more economical and reliable, are used beside the still conventional visual inspection by trained personnel.
It has been shown that transmitted light methods, in which the strip is passed between a light transmitter and a light receiver, do not meet the requirements at higher strip speeds. These strip speeds reach 10 m/min in modern production plants, in which the exposure time for the measurement is in a range of 150 microseconds. Also limits are placed on the on-line-defect testing by the useable light intensities.
Also it is often overlooked that the test light is introduced edge-wise into the strip and the strip is used as a light guide. The light is reflected repeatedly on the boundary surfaces of the strip with the surrounding atmosphere, as in a glass fiber used as a light guide.
A method of this type, for example, is disclosed in Japanese Patent Publication H 10-339705 of Dec. 22, 1998. According to this reference the defects to be detected in a glass panel continuously moving at about 4 m/min are measured. Light is coupled into both longitudinal edges of the panel and the resulting scattered light originating from the defect or fault is acquired or captured by means of vertical linear sensors arranged above and below the glass. Signals from the linear sensors are processed in an electronic analysis device. In order to exclude uninteresting particles adhering to the surfaces from the defect detection, the light should be introduced inclined to the side edges and conducted further from the outside to the inside by internal reflection at the boundary surface between the glass and the surrounding atmosphere. When using halogen lamps as the light source, that the intensity of the input test light strongly decreases, because of absorption at the center of the strip, for example with a 1.2 m width strip to 5% of the value at the edge, must be considered or taken into account. Thus during analysis an expensive computational compensation is required in order to correctly judge the size of the defects over the entire width of the edge from the acquired scattered light. The known method is limited to certain strip widths and strip speeds, because the required initial intensity of the input test light must be greatly increased with increasing strip width and speed and finally reaches an engineering limit.
In addition the known process presupposes trimmed side edges, because a definite input of the test light is not possible with the rounded edges and uncontrolled production conditions. Because the glass fractures or splits uncontrollably starting from the cut position in a direction opposite from the continuous motion direction and thus produces waste, the trimming of the long edges of the strip is not possible. Thus trimming cannot be used in online testing of a continuously moving strip.
Besides the known method also was not conceived for defect testing in a continuously moving strip, but for glass panels with trimmed long edges and a starting edge and an end edge, which are detected during forward motion by photocells and which are used to produce control signals for starting and stopping the testing process.